Replication of T4 DBA in vivo requires several proteins encoded by the phage DNA including the gene 43 DNA polymerase, the gene 32 DNA unwinding protein, the gene 44 and 62 DN-dependent ATPase, the gene 45 protein, and the gene 41 single-stranded DNA requiring nucleotidase. We have purified these proteins and have shown that they stimulate DNA synthesis in vitro with the T4 DNA polymerase with both native and single-stranded templates. We are presently studying the mechanism by which chains are initiated with circular single-stranded templates with these proteins, as well as mechanisms by which they accelerate the elongation of the primer. Using a new screening procedure, we have identified a temperature-sensitive E. coli mutant (mut 3) with a high mutation frequency (mutator) which maps at or very close to the pol C locus encoding DNA polymerase II, as well as a second mutation which greatly enhances the mutator activity of mut 3 and of mutants in pol C. We plan to search for additional mutants of these types and to determine the role of the proteins affected by these mutations.